This invention relates to an inkjet printing device for inks containing a high loading of pigment, comprising an ink-jet printhead for continuous printing, an ink reservoir, and a feeding circuit for feeding said printhead with ink from the reservoir and returning gutter ink from the printhead to the reservoir. The invention also relates to an inkjet printing process for inks having a high content in high density pigment.
There is an increasing interest in the printing of pigmented inks. However it is well known that when inks contain high density pigments the high density make them difficult to incorporate into inkjet inks. The Brownian motion of the particles in dilute solution is not sufficient to overcome gravitational forces and the particles tend to settle out of the ink and agglomerate. The continuous inkjet printing industry has battled against this problem for over ten years. It is still widely believed within this industry that in order for a pigmented ink to be successfully applied using ink-jet the dispersion must be stable. This requirement places very stringent demands on the ink chemistry, more particularly when inks containing a high loading of pigment or having formulations with room temperature viscosities in excess of 12 cPs or inks that contain security features such as fluorescent pigments have to be applied with continuous inkjet printing technology.
It is an object of the present invention to propose a solution to this problem.
Experiments made by the Applicant with this end in view, have shown that through the utilisation of the device according to the present invention an ink which has only a certain minimum degree of dispersion stability and which exhibits a phenomenon termed xe2x80x9csoft settlingxe2x80x9d can be successfully run and printed in a single nozzle continuous inkjet printer
A dispersion which exhibits soft settling is one in which the pigment settles out of dispersion on standing, but is readily redispersed by the application of mild agitation or shaking. A hard settling dispersion is one that cannot be readily redispersed after standing for a period of time.
The present invention therefore relates to an inkjet printing device for inks containing a high loading of pigment, comprising an inkjet printhead for continuous printing, an ink reservoir, and a feeding circuit for feeding said printhead with ink from the reservoir and returning gutter ink from the printhead to the reservoir, wherein the device further comprises on the one hand a two stages mixing arrangement comprising a recirculation loop with mixing means, taking ink from the reservoir and returning it to the reservoir, and a stirring system for ink contained in the reservoir, and, additionally, a means of heating the ink and ensuring the temperature of the ink is maintained at a predetermined temperature, above the ambient level.
According to a preferred embodiment, at least five static mixers are incorporated at strategic points within the system and the printhead feeding circuit comprises a filter placed between two static mixers, upstream of the printhead, and filter heating means arranged in such a manner that the ink temperature in the filter is higher than elsewhere in the printhead supply line. Further, a recirculation loop comprises a recirculation pump located between two static mixers.
According to a second aspect, the invention relates to a process for inkjet printing with inks formed of a dispersion of particles in a liquid.